0 to 999 Hz frequency counter

Question

I'm trying to build a frequency counter with a resolution of 1 Hz that will count any frequency between 0 and 999 Hz and hold that value. I've built the following in Multisim but I'm having trouble getting it to work.

The block diagram is the part inside the red perimeter...

Circuit in Multisim...

Here the frequency I want to display is the 120Hz signal. However, it only counts rather than directly displaying the 120Hz frequency and I've been able to get it to display up to 112 in Multisim after letting the simulation run for about an hour.

Also there is a bug in Multisim that prevents the crystal from working properly, which is why the digital clock is used to produce the 32.768kHz signal as show here:

UPDATE: I was able to get it to count to 234 and then reset to 000 with the following:

Can someone please tell me how to get it to hold the final count value?

Answer 1

You need a way to reset your counter before starting a new count.

The circuit below may suffice:

It uses the 4029 to generate the last bits of the divide down chain to produce the 0.5Hz signal. When 0.5Hz is low it enables the counters via their active low Clock Enable pin. When 0.5Hz goes high the counter stops and holds its value. Near the end of the 0.5Hz cycle the first 4029’s TC asserts. If we invert that we can use it to reset the counter to zero.

The 4543 includes a latch so if we generate a latch signal while the count is stable we’ll get a clean reading. AND’ing 0.5Hz and 1Hz together provide this latch signal.

EDIT I've added an LTspice simulation of the counter section (I don't have a 4543 model). 3.2seconds into the simulation the counter reads 124 as expected. After only 1 second it reads only 124*.9 = 112 because the pulse width of the gating 0.5Hz has an extra 8Hz tick at startup.

Use a NAND gate (for the 4543) if you want to see the count incrementing, or an AND gate if you just want it to latch the final output.

FINAL EDIT Here's an update with my final attempt:

The 4029 counters begin counting when 0.5Hz goes low (enabling their CE pin) and stop counting one second later when 0.5Hz goes low.

The counter outputs will remain stable while 0.5Hz remains high so we generate a LD signal during that time which will allow the counter value to be latched into the 4543s.

LD then deasserts so the 4543 now latch the captured value.

Towards the end of the 0.5Hz cycle the Terminal Count of the 4029 clock generator asserts and we use that to clear the 4029 counters (via the PE pins).

With this circuit the 7-segment displays will only show new values, updating every two seconds.

Answer 2

It sounds like a timing problem with the latch and reset signals. Years ago there was an article in Popular Electronics (Feb. 1980) about CMOS counter ICs that had a simple latch/reset circuit using a CD4011 quad NAND gate.

It's simply two differentiators that take the clock signal and create delayed pulses such that at the end of the counting period it would latch the final value into the display and then reset the counter. I used this in a digital capacitance meter and it worked well, I was even able to add a 555 timer to delay the reset pulse to cancel out any stray readings.

Perhaps this circuit could be adapted to your counter.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

Follow the chaining instructions found in the datasheet. the OR pattern looked good.

you will need to drive ~PE somehow from the 2Hz clock

you will probably want to drive LD from that clock also

Answer 4

Consider the MC14553B. This is a three-digit up-counter complete with an onboard latch and a multiplexed output for the displays. This could replace many of the individual ICs in your design.

https://media.digikey.com/pdf/Data%20Sheets/ON%20Semiconductor%20PDFs/MC14553B-Rev2006.pdf

Also, it is not clear how you want the circuit controlled. Is it supposed to count and display the input frequency just once, and freeze the display (waiting for some kind of reset)? Or count, latch the display, zero the counter, count the input, and update the display - continuously?